It is well known that components made from a semi-solid material possess great advantages over corresponding components produced in accordance with conventional processes. “Semi-solid” is referred to as a melt comprising a certain weight percentage of solid particles that have been generated upon cooling of the melt. The advantages of a cast component produced upon casting of such a material may be fewer defects, better mechanical properties, etc.
The production of metal components based on a semi-solid material normally includes the heating of a metal or alloy in a vessel to render it liquid, followed by the cooling of the molten material until it reaches a semi-solid state. Once the semi-solid state has been reached, the material may typically be cast in a mould or in a device for continuous casting for the formation of a product or a semi-product.
As they solidify, many metals and alloys are prone to form a so-called dendritic structure. However, since such structures have a negative effect on the thixotropic properties of the semi-solid material, they should be avoided if possible. According to the closest prior art, for example as disclosed in U.S. Pat. No. 6,645,323, such a formation of a dendritic structure upon cooling and solidification is avoided by means of agitation of the melt.
According to U.S. Pat. No. 6,645,323 the liquid, molten metal, is rapidly cooled under controlled conditions while it is agitated by rotating mechanical devices to form a desired thixotropic slurry. Other ways of inducing the agitation, for example by means of an electromagnetic stirrer, are also feasible. The agitation continues up to a certain point when a predetermined, small, fraction of solid material has been formed in the melt. Then the cooling continues without further agitation. When a given fraction of solid metal is obtained in the slurry, it is used in a casting operation.
However, the process according to this prior art needs external cooling of the melt, either by a cooling means provided on the outside of the vessel or by a cooling means provided in the melt, for example in the stirrer. Accordingly, prior art requires a control of the cooling, including temperature control, for the purpose of controlling the obtained fraction of solid material. This makes these prior art methods relatively slow and costly.
Prior art also teaches the addition of a solid metal or alloy to a melt, either as an inoculant for the promotion of nucleation or as an alloying means.
WO 2004027101 discloses a method for refining of primary silicon in hypereutectic alloys by mixing a hypereutectic alloy and a solid/semi-solid hypoeutectic alloy. The method provides control of the morphology, size, and distribution of primary Si in a hypereutectic Al—Si casting by mixing a hypoeutectic Al—Si liquid with one that is hypereutectic to impart desirable mechanical properties due to the formation of the primary Si particles. According to this prior art, the method also requires a control of the cooling of the hypereutectic alloy-hypoeutectic alloy mixture for a length of time to form a semi-solid metal. The generally uniform distribution of primary Si particles is controlled by a more rapid drop in temperature during mixing. No stirring of the melt during cooling thereof is suggested.
According to U.S. Pat. No. 6,880,613, a method for the refining of primary aluminium in hypoeutectic alloys by mixing at least two hypoeutectic alloys into a solid/semi-solid hypoeutectic slurry is described. The method provides control of the morphology, size, and distribution of primary Al in a hypoeutectic Al—Si casting by mixing a hypoeutectic Al—Si liquid with solid hypoeutectic Al—Si particles to impart desirable mechanical properties. In one embodiment of this prior art, small solid chunks of hypoeutectic Al—Si alloy was used to mix with liquid hypoeutectic Al—Si alloy to form a hypoeutectic Al—Si slurry. The generally uniform distribution of primary Al particles is controlled by a more rapid drop in temperature during mixing. No stirring of the melt during mixing is suggested.